Passive optical networks (PONs) are point-to-multipoint networks, in which a central optical line terminal (OLT) is connected with a plurality of optical network units (ONUs) by means of an optical distribution network (ODN) including spans of optical fiber connected by optical splitters and couplers (OSCs). Typically, a single OLT controls the communication with all ONUs installed in a PON, broadcasting downstream signals to the ONUs, and organizing upstream communication from the ONUs to the OLT using time-domain multiple access (TDMA). In TDMA, each ONU is assigned a time slot, within which the ONU may transmit an upstream signal. The OLT is configured to ensure that upstream transmissions from different ONUs do not collide, that is, do not occupy a same time slot. Due to the TDMA organization of a PON, upstream transmissions from ONUs to the OLT are usually bursty in nature, while the downstream transmissions from the OLT to ONUs tend to be more continuous. A technician servicing a PON travels to various locations of the ODN, and checks optical power levels at those locations, to ensure that optical signals propagate to appropriate ODN destinations with acceptable optical loss. Both average optical power levels and peak optical power levels may be measured and recorded. Peak optical power levels may be useful for characterization of bursty upstream signals.
Referring to FIG. 1, a prior-art PON test device 100 is shown. The PON test device 100 was disclosed by Ruchet in US Patent Application Publication 2006/0171711 A1. The PON test device 100 includes a 2×2 coupler 32, a wavelength division multiplexor (WDM) 68, and first 38, second 42, and third 44 photodetectors for detecting optical signals at wavelengths of 1310 nm, 1490 nm, and 1550 nm, respectively.
In operation, the test device 100 is coupled in an optical path between an OLT 110 and an ONU 120. The OLT 110 generates downstream optical signals S2 at 1490 nm and S3 at 1550 nm, which are coupled to a first through port 28 of the 2×2 coupler 32. The 2×2 coupler 32 power-splits the downstream optical signals S2 and S3. Eighty percent (80%) of optical power of the downstream optical signals S2 and S3 are coupled to a second through port 30 of the 2×2 coupler 32. Then, the signals S2 and S3 (attenuated by 20% by the 2×2 coupler 32) propagate to the ONU 120. Twenty percent (20%) of the optical power of the downstream optical signals S2 and S3, denoted in FIG. 1 as S2′ and S3′, are coupled to a first drop port 36 of the 2×2 coupler 32. The first drop port 36 is coupled to the WDM 68. The WDM 68 separates the signals S2′ and S3′, directing the resulting signals S2″, S3″ to the second 42 and third 44 photodetectors, respectively.
The ONU 120 generates an upstream optical signal S1 at 1310 nm, which is coupled to the second through port 30 of the 2×2 coupler 32. The 2×2 coupler 32 power-splits the upstream optical signal S1. Eighty percent (80%) of the optical power of the upstream optical signal S1 is coupled to the first through port 28 of the 2×2 coupler 32, and the attenuated signal S1 propagates to the OLT 110. Twenty percent (20%) of the optical power of the upstream optical signal S1, denoted in FIG. 1 as S1′, is coupled to a second drop port 34 of the 2×2 coupler 32. The split signal S1′ is coupled to the first photodetector 38. As a result, the optical power of the signals S1, S2, and S3 propagating between the OLT 110 and the ONU 120, may be measured without breaking an optical link between the OLT 110 and the ONU 120.
In a PON, a large number of ONUs may be connected to a single OLT. When the PON is expanded or reconfigured, some ONUs remain connected to the ODN, and some ONUs are transferred to be connected to different optical fibers within the ODN. Due to a high reconfiguration rate of the PON, and due to a large number of optical connections within the PON, network operators are increasingly facing a problem that network documentation is not synchronized with a current configuration of the PON, making network servicing difficult. It is not uncommon for a service technician to call the central office for a network update, and/or disconnect individual subscribers successively one by one, in an attempt to find a correct optical fiber to take an optical power measurement. Besides being tedious and prone to misconnection errors, this procedure is disruptive to subscribers.